(1) Field of the Invention
The present invention relates to a water-in-oil emulsion explosive composition (hereinafter, abbreviated as W/O emulsion explosive composition), and more particularly relates to a W/O emulsion explosive composition containing a specifically limited emulsifier, which forms W/O emulsion, and having an improved storage stability in initiation sensitivity in a small diameter cartridge (25 mm) and at low temperatures and further having an improved resistance against dead pressing.
(2) Description of the Prior Art
W/O emulsion explosive compositions have hitherto been investigated. Recently, there have been proposed several W/O emulsion explosive compositions having an improved initiation sensitivity in small diameter cartridges (capable of being detonated by a blasting cap) without containing any explosive sensitizers, such as nitroglycerine and the like; non-explosive sensitizers, such as monomethylamine nitrate and the like; and auxiliary sensitizers, such as detonation catalyst, sensitive oxidizer and the like.
For example, U.S. Pat. No. 4,110,134 discloses that a W/O emulsion explosive composition, which contains sorbitan monooleate sold under the trademark of Glycomul "O" as an emulsifier and further contains glass microballoons as a gas-retaining agent, is completely detonated (explosive temperature: 21.1.degree.-26.7.degree. C.) up to a density of maximum 1.25 in a cartridge diameter of about 1.25 inches (31.8 mm) by a No. 6 blasting cap after 18-24 hours from the production of the explosive composition. (The inventors have found out by a gaschromatography that Glycomul "O" is actually, as described later, a mixture of sorbide oleate, sorbitan oleate and sorbitol oleate in a mixing ratio of sorbide oleate/sorbitan oleate/sorbitol oleate of about 25/68/7 in weight basis, each of the esters being a mixture of mono-, di- and tri-esters.)
Japanese patent laid-open specification No. 188,482/82 discloses that a W/O emulsion explosive composition, which contains, as an emulsifier, monolaurate, monoisostearate, monolinoleate, dioleate, trioleate or tetraoleate of sorbitol (each of these fatty acid esters of sorbitol is not used in the form of a mixture of mono-, di- and tri-esters, but is used in the form of a single compound) can be completely detonated at -5.degree. C. by a No. 6 blasting cap even after 21-33 temperature cycles, in each of which cycles the explosive composition is kept at 60.degree. C. for 24 hours and then at -15.degree. C. for 24 hours, and which 21-33 cycles correspond to about 21-33 months of storage at room temperature (10.degree.-30.degree. C.).
However, when the W/O emulsion explosive compositions containing the emulsifiers of the above described U.S. patent and Japanese patent laid-open specification were subjected to a temperature cycle test, which was carried out by the inventors of the present invention and can indicate relatively correctly the actural storage life of the explosive compositions at room temperature (0.degree.-30.degree. C.), and wherein such a temperature cycle that a sample explosive composition is kept at 60.degree. C. for 24 hours and then at -15.degree. C. for 24 hours, is repeated until the sample explosive composition is no longer detonated at -5.degree. C. by a No. 6 blasting cap (one cycle corresponds to about one month storage at room temperature), the W/O emulsion explosive composition of the U.S. patent and that of the Japanese patent laid-open specification were completely detonated after 19 and 29 cycles (corresponding to 19 and 29 months), respectively. That is, both the W/O emulsion explosive compositions are satisfactory in storage stability for the use in Japan. However, W/O emulsion explosive is inherently thermodynamically unstable, and is broken due to the lapse of time and other various reasons. As the result, W/O emulsion explosive is decreased in its initiation sensitivity and is not finally detonated by a blasting cap. Among others, the most serious phenomenon is the non-explosion phenomenon (generally called as "dead pressing phenomenon"), which is caused by the breakage of emulsion due to the actions of preceding shock wave from pre-explosion in a same bore hole, shock wave from pre-explosion in an adjacent bore hole, combustion gas and the like. When the stability of W/O emulsion and the above described non-explosion phenomenon (dead pressing phenomenon) are taken into consideration, the above described storage lives of 19 and 29 months are still unsatisfactory, and the development of a W/O emulsion explosive composition having a high storage stability in initiation sensitivity in a small diameter cartridge (25 mm diameter) and at low temperatures and having a high resistance of W/O emulsion against dead pressing has been eagerly demanded.
The inventors have made various investigations for a long period of time by taking the above described problems into consideration, and have found out that a W/O emulsion explosive composition containing, as an emulsifier, a mixture of sorbide fatty acid ester, sorbitan fatty acid ester and sorbitol fatty acid ester in a specifically limited mixing ratio, which has never hitherto been known, of the esters has a very superior performance in the storage stability in initiation sensitivity in small diameter cartridges and at low temperatures and further in the resistance against dead pressing to the performance of W/O emulsion explosive compositions containing a conventional emulsifier. As the result, the present invention has been accomplished.